This invention relates to telecommunications equipment which terminates subscriber lines and supports high speed data service for subscribers. The invention is especially suited, but not limited to, providing high speed data services which exceed the capability of conventional POTS terminating equipment.
In the United States, subscribers are commonly provided with telephone services known as plain old telephone services (POTS). Such services include providing conventional dial-tone and automated dialing features including the use of dual frequency tone signaling to communicate dialed number information. Additional modem telephone features including conferencing, call waiting, incoming caller identification, and other commercially available features.
Subscribers utilize a POTS line to carry conventional modem signals controlled by a personal computer to another modem via the public switch telephone network. Modems are currently available which advertise data communication rates up to 33.6 kilobits per second over dial-up analog subscriber lines. New modems have been announced which are to be capable of data communications up to 56 kilobits per second. Those skilled in the art will understand that the maximum data rate which can be transmitted using modems over a POTS subscriber line is limited by the sampling rate, the bandwidth of the channel, and the rate by which the analog signals are converted to digital signals by the line card units which terminate each POTS line at a central office. Thus, conventional modem data rates are limited by the POTS line cards which terminate the subscriber lines.
It is well known that subscribers in the United States can lease special subscriber lines from their telephone providers which accommodate higher data rates than are supported by conventional dial-up subscriber lines. Such higher speed lines utilize different terminating circuitry at the central office to accommodate higher data rates. For example, integrated services digital network (ISDN) terminating equipment will provide a subscriber with the capability of higher data rates than a conventional POTS terminated line. The known asynchronous digital subscriber line (ADSL) technique which is supported by available equipment provides a significantly higher data rate to a subscriber over a conventional copper two-wire subscriber line. Of course, the ADSL service requires proper terminating equipment at the central office and at the consumer premises to accommodate the greater throughput capabilities, i.e. data rates.
FIG. 1 illustrates currently available services in the United States to subscribers using conventional two wire copper loops. Equipment to the right of dashed line 10 represents customer premises equipment (CPE); equipment to the left of line 10 represents central office line termination equipment. The POTS line interface 12, also known as a line card, provides an interface between digital incoming and outgoing communication lines 14 and 16, and analog signals carried on subscriber line 18. For example, lines 14 and 16 may carry 64 kilobit per second pulse code modulation (PCM) signals representative of analog information received from and transmitted to line 18. A main distribution frame (MDF) 20 is used to interconnect a plurality of incoming subscriber lines to various terminating equipment at the central office. In this example, line 18 is connected through a diplex filter 19 and MDF 20 to subscriber line 22. A conventional telephone instrument 24 at the consumer's premises is connected through diplex filter 23 to subscriber line 22.
FIG. 1 also illustrates another service to the subscriber which provides a high speed data capability. An ADSL interface circuit 26 provides an interface between the central office and the subscriber for transmitting and receiving data at rates up to several Megabits per second. Lines 28 and 30 provide inbound and outgoing digital data communications representative of information to and from the subscriber carried on line 32. Line 32 is connected via diplex filter 19 and MDF 20 to subscriber line 22. An ADSL interface 36 provides an interface between conventional digital data communicated with a user's personal computer 38 and ADSL analog format signaling communicated on line 37. The diplex filter 23 couples the ADSL signal between ADSL interface 36 and subscriber line 22. The advantage to the user is that the ADSL facilities support a substantially higher data rate than would be available if the subscriber utilized communications terminated via the POTS line interface 12.
A disadvantage illustrated in FIG. 1 is that diplex filters 19 and 23 are required. These known diplex filters function to separate the higher frequency signals associated with ADSL signaling from the lower frequency signals (typically &lt;4 Kilohertz) associated with the conventional POTS communications. The conventional diplex filters consist of a lattice of inductors and capacitors that provides the needed filtering while maintaining direct current (DC) continuity required to support battery feed current for POTS service. Because of the DC which must be carried and the voltages which are encountered, such as during ringing, the inductors used in the diplex filters occupy a relatively large volume. Thus, a conventional diplex filter occupies a significant amount of space, for example about 90 cubic centimeters. The size of these filters and the wiring associated with connecting them to the MDF, ADSL interface, and the POTS line interface take up precious space in the terminating equipment cabinets where space is limited. This problem is magnified as more and more subscribers request such service. Therefore, a need exists to provide such high speed services while minimizing the space and wiring requirements especially at the terminating equipment.